When an electric power station breaks down due to an unexpected accident or when a number of wind farms or photovoltaic power plants that have been interconnected are separated off, the associated electric power system is thrown into a state of emergency because its electric power supplying capability abruptly drops.
There has been proposed a technology for coping with such an emergency by discharging energy storage devices (e.g., stationary storage batteries or secondary batteries on electric vehicles) that are installed at various customer locations to manage the balance of electric power supply and demand during a period of time ranging from several to several tens of minutes until the output of the existing electric power station is changed and until another electric power station becomes operational (see Non-patent documents 1, 2, and 3). The energy storage devices that are installed at various customer locations will hereinafter be referred to as “ES” (Energy Storage).
According to the technology which manages the balance of electric power supply and demand by discharging the ESs installed at various customer locations, a number of ESs that are in widespread usage among customers can be used to manage the balance of electric power supply and demand. The widespread usage of ESs makes it possible to increase the capacities of batteries (batteries provided by ESs) that are used to manage the balance of electric power supply and demand.
If a technology wherein “each ES monitors the frequency of electric power at a point of junction to the electric power system and autonomously discharges itself in proportion to a frequency shift to keep the frequency of electric power within a desired range” is employed, then it becomes possible to make the battery respond quickly in case of emergency.
FIG. 1 is a diagram illustrating the technology wherein “each ES monitors the frequency of electric power at the point of junction at the electric power system and autonomously discharges itself in proportion to a frequency shift to keep the frequency of electric power within a desired range. The point of junction at the electric power system may, for example, be the outermost portion of a wire through which electric power is applied from the electric power system through an electric power meter to a power switchboard on the customer's premises. However, on the assumption that the frequency remains the same on the customer's premises, the point of junction at the electric power system may be a point of junction where the ES is wired.
In FIG. 1, controller 102 that controls ES 101 monitors the frequency of electric power at system junction point 103. Controller 102 controls a discharged level of ES 101 depending on a value produced when the monitored result (measured frequency) is subtracted from the nominal frequency (e.g., 50 Hz) of electric power.
In FIG. 1, a unit that is indicated as ES 101 includes a storage battery pack, a storage battery control unit, and a power conditioner for converting a DC output from a storage battery into an alternating current, which are omitted from illustration for the sake of brevity.